The present invention relates to a hydraulic power steering system for a vehicle, in particular for a motor vehicle.
The known hydraulic power steering systems essentially comprise a constant displacement pump, operated by a thermal engine of the vehicle and comprising a suction port and a pressurized oil delivery port, an oil reservoir connected to the pump suction port, a double-acting hydraulic cylinder capable of controlling direction of the vehicle front wheels, and a distribution unit controlled by a steering wheel of the vehicle and comprising an inlet port connected to the pump's delivery port, a vent port connected to the fuel tank and two service ports connected to the respective chambers of the hydraulic cylinder.
The pump comprises a pumping unit and a valve for adjusting the flow rate from said pump to prevent oversizing of pipes and difficult checking of the oil flow circulating in the system. Said valve is designed so as to allow a constant oil flow rate to be displaced towards the distribution unit, equal to the flow rate pumped by the pumping unit at a rotation speed corresponding to the thermal engine's minimum r.p.m.; the difference in the flow rate produced by the pumping unit at higher rotation speeds being circulated back into the pump.
The distribution unit further comprises an outer distributor, rotating about its axis and controlled by the hydraulic cylinder through a rack-pinion coupling, and an inner distributor, connected to the steering wheel and rotating with the latter about its axis with respect to the outer distributor, to unbalance the oil pressures in the outlet ducts and activate the hydraulic cylinder; the latter causes the outer distributor to rotate, tending to follow the inner distributor's rotation, so as to cancel the angular displacement between the said distributors and, consequently, oil flow from and to the cylinder chambers, upon the steering wheel reaching a static position.
However, the systems of the type briefly described hereinabove have a number of drawbacks.
Pump displacement must be so designed, that both oil delivery and pressure in the hydraulic power steering system can be such, as to guarantee good vehicle maneuverability in the maximum performance, for example when quickly steering to park the vehicle from a still condition thereof and minimum r.p.m.
Therefore, it is clear that in no further condition of use is the oil volume displaced by the pump exploited, thus causing energy dissipation. In particular, when the vehicle is travelling on motorways at high speeds, with minimal steering wheel movements, the energy dissipated by the hydraulic power steering system is quite high, in that the pump is dragged by the engine at a higher rotation speed, with respect to the engine's minimum r.p.m., thus causing large oil flows to be delivered by the pump and mostly circulated back into the pump itself.
Furthermore, the difference between the power absorbed by the pump and the power used by both distribution unit and hydraulic cylinder is continuously dissipated and converted into heat, causing oil overheating problems, as well as limited life of both hydraulic system components and oil.
Finally, said overheating favors noise problems, during lamination of oil flow rate in the system.